Light duty automatic door closers are well known and are usually spring biased to hold a door closed while allowing it to be manually opened as desired against the bias of the spring, which immediately acts to reclose the door when the manual opening force is released. Powered automatic door operators, on the other hand, are usually operated automatically at high speed ahead of traffic by traffic sensing controls, and have required heavy duty drives and relatively complex controls for timely operation and safety.
There is a felt need for a powered light duty door operator which falls between the relatively inexpensive aforementioned spring biased operators and the relatively expensive automatic high speed powered operators also aforementioned. Such a powered light duty operator would be intended for operation similar to the spring biased operator, i.e., it would be manually opened by traffic passing through, and would automatically close behind the traffic. However, such a powered light duty operator would also have the ability to both open and close the door under power controlled by suitable electrical switches, particularly for use by handicapped people in wheelchairs, and the door would be operated very slowly under power as compared to the usual high speed automatic powered door operator.
A prior art light duty powered door operator recently introduced by the Stanley Works, Farmington, Conn., under the trade name "Silent Swing" has a stallable alternating current electrical gear motor drive for selectively driving a door in opening and closing directions to full open and full closed positions with resilient damping at each end position. Power remains on the motor at full open and closed positions, consuming about 40 watts of electrical power. The door may be opened or closed against the drive, or the drive may be speeded up or stalled by manually assisting or resisting the door. Since the motor drives the door directly through appropriate gears and linkage, the motor and gear drive must be relatively heavy duty apparatus in order to stand the abuse when the door is abruptly manually stopped, reversed in direction, or drastically oversped. There is no inherent limit to such abuse except manual strength limitations, and due to the high gear ratios involved, very high motor speeds and inertia forces may be generated by fast traffic bumping the door. This operator is intended to be controlled by a simple double-pole-double-throw toggle switch for relatively slow operation (Approximately 3.5 seconds for full travel) thereby probably assuring that normal traffic would just push the door open against the mechanism at a much faster and potentially damaging rate (as in the aforementioned spring biased door closer), leaving the door operator to close the door only. Probably handicapped persons, whether in wheelchairs or afoot, would be the only traffic using the operator to open the door.
An earlier Stanley Works powered door operator employed a slip clutch between a power drive element and an operating linkage, but the drive element was powered only to fixed limits of its own movement, and clutch slippage necessitated resetting the clutch so that the door would not remain partially open until a subsequent operation. Such an arrangement was presumably intended to be for emergency use only, as in case of fire or power failure, or a person caught by a closing or opening door.
In contrast, the present invention, in its preferred embodiment, is powered by a very small, permanent magnet type, direct current motor of about 0.025 rated horsepower. The motor may be stalled for indefinite periods without drawing excessive current or overheating since it is operated at greatly reduced voltage and at about its full load rated current when stalled. Its stalled torque is therefore relatively low while running torque decreases generally nearly linearly from stall torque to full no-load speed. To generate the force required for even relatively slow door operation (about 6 seconds for full travel), five stages of gear reduction (a ratio of about 500:1) are employed between the motor and the output shaft of the drive unit. This allows use of a relatively small and inexpensive commercially available standard drive unit which may be controlled readily with equally small and relatively inexpensive solid state electronics. These electronic controls in turn allow use of the small inexpensive drive unit by making it possible to protect it from physical abuse through use of a slip clutch, as well as providing advantageous operational characteristics and capabilities which have not been technically or commercially practical with any known prior art devices. The preferred embodiment of the present invention provides for almost negligible power consumption, operation similar to a spring biased door closer for manual operation, switch controlled powered door opening and closing when desired, and manual overriding operation either with or against the power drive without danger of excessive overspeeding or shock-loading the drive unit destructively. The present door operator is economically practical as an improved substitute for the conventional spring-biased door closer in view of government-mandated requirements for power operation of such doors to accommodate the handicapped.